While aromatic polyesters are almost totally resistant to microbial attack, most aliphatic polyesters are biodegradable due to their potentially hydrolysable ester bonds. Polyhydroxyalkanoates or PHAs are linear polyesters produced in nature by bacterial fermentation of sugar or lipids and hence are considered renewable bioplastics. These bioplastics are produced by bacteria to store carbon and energy. More than 150 different monomers can be combined within this family to give materials with extremely different properties. These polymers are biodegradable. Poly(3-hydroxybutyrate) or P(3HB) is the best well-known and most used member of the class of PHAs. It was discovered by Lemognie in 1925 in the bacteria Alcaligenis euterophus, in which, under optimal conditions, above 80% of the dry weight is of P(3HB). PHAs with short side chains, such as polyhydroxybutyrate (PHB), a homopolymer of 3-hydroxybutyric acid units, are crystalline thermoplastics; PHAs with long side chains are more elastomeric. PHAs of microbial origin containing both 3-hydroxybutyric acid units and longer side chain units from C5 to C16 are also known. A number of bacteria which produce copolymers of 3-hydroxybutyric acid and one or more long side chain hydroxyacid units containing from five to sixteen carbon atoms have been identified. A known example of specific two-component copolymers includes PHB-co-3-hydroxyhexanoate. Other biodegradable polymers are polylactic acid (PLA), polycaprolactone (PCL), polybutylene succinate (PBS), polyanhydrides, polyvinyl alcohol (PVA), most starch derivatives, and cellulose esters like cellulose acetate and nitrocellulose and their derivatives (celluloid). In this application PLA and PCL are not considered PHAs since PLA is produced chemically from lactic acid or lactide, and PCL is produced starting from fossil fuel. PHAs are considered fully compostable, meaning they will biodegrade under common composting conditions. These biodegradable polymers are typically used for disposables.
US 2009018235, for instance, refers to a polymeric composition prepared from a biodegradable polymer defined by poly(hydroxybutyrate) (PHB) or copolymers thereof, and at least one other biodegradable polymer, such as polycaprolactone (PCL) and poly (lactic acid) (PLA), so as to alter its structure. The composition further comprises at least one additive of the type of natural filler and natural fibers, and, optionally, nucleant, thermal stabilizer, processing aid, with the object of preparing an environmentally degradable material.
US 2009030112 describes a biodegradable polymeric composition for manufacturing biodegradable articles and films, that comprises PHB, plasticizer obtained from a renewable source, nucleant additive, flow aid additive and a thermal stabilizer additive.
EP 781309 A and CA 2231568 both relate to polymeric compositions that are biodegradable and that can be melt processed into various forms, including films, fibers, and nonwovens. The compositions include compatible or semicompatible blends of biodegradable polymers and have physical and thermomechanical integrity. Films formed from preferred polymeric compositions are suitable for use as backsheets in disposable absorbent articles. In a preferred embodiment, the polymeric composition includes a polyhydroxyalkanoate and at least one other biodegradable polymer selected from aliphatic polyester-based polyurethanes, a polylactide, polycaprolactone, or a mixture of two or more of these polymers.
Biodegradable compositions further comprising PLA and/or starch or similar biodegradable polymers have been used particularly for the manufacturing of disposables. On the other hand, PHAs have also been used in non-disposables or durable goods, as replacement for fossil or petrochemical based polymers. In such applications being made and accordingly being labelled as made a composition from a renewable source is highly appreciated. Biodegradability for non-disposable and/or durable goods is then a disadvantage. For durable goods, the presence of degradability enhancing components like PLA and starch are clearly undesired. PLA, being rather crystalline in nature, is undesired also for its lack of thermal stability and rather poor processability.
WO 2011/007092 relates to a PHA composition, further including: (A) a core-shell elastomer compound; and (B) an olefin copolymer including an ethylenic monomer having an epoxy function. Said composition exhibits excellent impact properties, in particular under cold conditions. The invention also relates to a method for manufacturing said composition and to parts manufactured from said composition. Component (B) may for instance be Lotader® AX 8900 (see Examples), an acrylic type terpolymer. On the other hand, the compositions are based on PLA and therefore not an ideal replacement of conventional plastics.
EP1826241 also discloses a resin composition comprising an aliphatic polyester type biodegradable polymer and a copolymer of the core-shell type comprising an acrylic rubber as the core layer and a vinyl-monomer-derived polymer as the shell layer. The biodegradable polymer may be a PHA. The acrylic rubber comprises an alkyl acrylate co-polymer which may also comprise aromatic vinyl monomers. The compositions are exemplified in experiments with several PHAs and a core-shell graft copolymer comprising an acrylic rubber as the core layer and a vinyl monomer-derived polymer as the shell layer (Kane Ace M-400, from Kaneka). Although the copolymer improve some of the mechanical properties, there is still room for further improvement.
EP 0701586 A discloses a polyester composition that comprises a biodegradable polyester and a plasticising quantity of a particular plasticizer.
Although it is known that polyhydroxybutyrate (P3HB) has properties very similar to polypropylene (PP), it is more crystalline than PP and typically has a lower tensile strength and lower elongation at break. It is therefore not as easy to melt process into consumer articles, and the articles so produced are more fragile. On the other hand, there is an increasing demand for bioplastics and compositions based thereon.
Vincotte is an organisation specialized in certifying biodegradable products. As a result of the increased environmental awareness among customers, there is a growing market for products on a basis of renewable raw materials. That environmentally conscious motivation on the part of customers is exactly the reason why there is a need for an independent, high-quality guarantee of the renewability of raw materials. Vincotte therefore proposes a single to four star “OK biobased” certification system, that provides information on the content of renewable materials in the labelled product.
Many companies have tried to prepare compositions that contain more than 20% (by weight) of non-fossil carbon (single star “OK biobased”), preferably more than 40% wt of non-fossil carbon (double star “OK biobased”) and that can substitute the common PP based compositions for durable mass-produced goods such as mobile phone parts. Of importance, such compositions should have good thermal and mechanical properties. For instance, it should have an elongation at break (ASTM D638) of greater than 3%, preferably greater than 3.5%, more preferably greater than 4%, an impact resistance (ISO179 1 eU, 23° C., unnotched) of greater than 18 kJ/m2, and a flexural modulus (ASTM D790) of greater than 950 MPa, preferably greater than 1000 MPa, more preferably greater than 1500 MPa. Such thermal and mechanical properties are important for the production of durable goods and for the properties of these durable goods.
Unfortunately, so far no compositions that contain more than 20% wt of non-fossil carbon have been found that can meet the demands both of the producers and the end-users. It is therefore of interest to find a composition that can be used as a PP substitute in such applications.
Moreover, it would be of interest to find compositions that have improved processing properties. For instance, a common PP composition may have a melt flow index (MFI, ASTM D1238 @ 230° C./2.16 kg) of 26 g/10 min. Achieving a similar MFI at a lower temperature will allow milder injection moulding conditions and thus will provide some energy savings while processing the composition. Finally, it would be of interest to find a composition that shows better aesthetical properties (gloss), improved ability to be printed, painted or coated (polypropylene requires a special treatment), and improved UV stability. Such compositions have now been found.